“The way I see it, why does one need to appeal to the supernatural when there are real world metaphysical reasons for acting in a moral way?”
What are the “real world metaphysical reasons for acting in a moral way”?
They're innumerable. One of the most important would be that hurting other people and causing suffering violates my conscience, and makes it more likely that those things will be visited on myself, my family, and my country.
A guy told me the other day that he could think of no reason to tell me not to murder someone without appealing to the supernatural. Maybe you're of the same mind as he.